Monolithically integrated InGaN/GaN quantum nanowire devices

ABSTRACT

InGaN/GaN quantum layer nanowire light emitting diodes are fabricated into a single cluster capable of exhibiting a wide spectral output range. The nanowires having InGaN/GaN quantum layers formed of quantum dots are tuned to different output wavelengths using different nanowire diameters, for example, to achieve a full spectral output range covering the entire visible spectrum for display applications. The entire cluster is formed using a monolithically integrated fabrication technique that employs a single-step selective area epitaxy growth.

This application is a continuation application of U.S. patentapplication Ser. No. 16/912,372, filed Jun. 25, 2020, which is acontinuation of U.S. patent application Ser. No. 15/629,715, filed AprilJul. 21, 2017, now U.S. Pat. No. 10,734,545, both of which are herebyincorporated by reference in their entireties.

FIELD OF THE DISCLOSURE

The present disclosure relates to solid-state semiconductor devices and,more specifically, to semiconductor-based quantum nanowire devicesformed of InGaN/GaN quantum structures.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

Driven by the need for smaller size, reduced power consumption, andenhanced efficiency and functionality, future ultrahigh resolutiondisplay technologies require the development of submicron-scale,high-efficiency, multicolor light sources monolithically integrated on asingle chip. The challenges of organic light emitting diodes (OLEDs) forthese applications include limited lifetime of organic materials,relatively expensive manufacturing process, low efficiency andbrightness, and poor stability. Moreover, it has remained difficult toachieve micro-scale or nano-scale devices using organic materials.

GaN-based quantum well Light Emitting Diodes (LEDs) are bright, stableand efficient, but usually only emit in one color. It has also remaineddifficult to achieve efficient deep green and red emission usingGaN-based quantum well LEDs. Additionally, there is no establishedtechnology to spatially vary Indium (In) compositions in quantum wellsto achieve multicolor emission on the same substrate.

Recent studies have shown that such critical challenges can bepotentially addressed by using InGaN nanowire structures. Nanowire LEDheterostructures exhibit low dislocation densities and high lightextraction efficiency. GaN-based nanowire LEDs and lasers operating inthe ultraviolet (UV), blue-green, and red wavelength range have beendemonstrated.

Furthermore, it has been shown that multicolor emission can be achievedfrom InGaN nanowire arrays monolithically integrated on a single chip.It is further envisioned that display technologies based on pixels ofsingle nanowire LED arrays integrated on the same chip represent theultimate light sources for the emerging three-dimensional (3D)projection display, flexible display, and virtual retinal display (VRD)technologies. The radiation pattern and emission direction can bewell-controlled and tailored by the columnar structure of each singlenanowire, which is essential to achieving ultrahigh definition displays.In addition, pixels of single nanowire-based LED arrays can be much moreefficient in heat dissipation and can operate at extremely largeinjection current levels. Critical to these technology developments isthe demonstration of full-color, tunable light sources including LEDsand lasers using single, or a few nanowires on the same chip. Thisrequires a precise tuning of alloy compositions in different nanowirestructures and that these compositional variations should be ideallyintroduced in a single growth/synthesis step.

It was previously demonstrated in prior art that it was possible toproduce multiple colors by varying the diameters of nanowires that arein a densely packed array in a one-step selective area epitaxy, that is,without changing the global growth parameters of the crystal growthprocess and system. However to make arrays, on the same chip, that emitin multiple colors the process had to be done in a multi-step selectivearea epitaxy process. This approach, as used in the fabrication ofdensely packed nanowire arrays, takes advantage of the shadowing effectof neighboring nanowires to alter the InGaN composition in denselypacked array of nanowire structures. To date, however, little is knownabout the mechanism on how to controllably vary the alloy compositionsat the single nanowire level without changing the global growthparameters. The monolithic integration of multicolor, single nanowireLEDs on the same chip has thus remained elusive.

It is desirable, and extremely challenging, to be able to fabricate asemiconductor-based light-emitting photonic device that has all thefollowing attributes. Successful realization of a product that truly hasall the characteristics which are listed below has been one of the holygrails of the photonics industry for several decades now. These deviceshave many applications in various products such as for examplehigh-resolution and true-color displays that are used in applicationssuch as computer screens, mobile phone screens, and high-definitiontelevisions.

SUMMARY OF THE INVENTION

The present techniques include methods of fabricating solid-statesemiconductor devices formed of nanowires, monolithically integrated asa single, repeatable cluster. The fabrication techniques are able toform these clusters having nanowires of different diameters, eachcapable of emitting at a different peak wavelength, such that the entercluster is able to provide an output over a range of frequencies, suchas over the entire visible spectrum. Each cluster may be formed as asingle, repeatable chip like structure that may be fabricated in largescale applications, for example to form a digital display having a largearray of these clusters.

The nanowire devices herein are based on a Gallium Nitride (GaN)material system, the crystalline structure of which can be grown oneither a Silicon (Si) or Sapphire substrate which is a type of Aluminumoxide mineral. The nanowires are formed of quantum structures formed ofIndium Gallium Nitride (InGaN) and Gallium Nitride (GaN) layers, i.e.,InGaN/GaN layers. The InGaN/GaN layers form the active region of thenanowire structures and are quantum layers that may be formed of quantumdots, quantum disks, quantum arch-shaped structures, quantum semi-polarplanes, quantum wells, quantum dots with a shell, or other similarquantum structures or combination thereof. As electrons combine withholes in the active region of each nanowire, photons are generated atspecific wavelengths. The precise wavelength of the emitted photonsdepends on the specific composition of the InGaN/GaN quantum layers, aswell as the shape and of the nanowire.

As the inventors have found, with the structural and optical propertiesof InGaN/GaN quantum layers depended upon nanowire diameter, a uniquemonolithically integrated fabrication may form InGaN/GaN nanowire lightemitting diode (LED) devices that have a collective emission wavelengthtunable across an entire spectral range (e.g., across a visible spectralrange). By controlling the diameters of the nanowire structures and thepercentage composition of In, Ga, and N in the respective InGaN/GaNquantum layers of the nanowires, it is now possible to engineer acluster that emits over a tunable wide spectral range, by selecting theemitting wavelengths of each individual nanowire. Moreover, themonolithically integrated fabrication allows for precise control overthe spacing between nanowire structures during fabrication, where thatspacing, for the first time, allows for not only nanowires of differentdiameters, but also for the precise control over composition of elementsin the quantum active regions, which otherwise would make it impossibleto form a cluster having a wide emission spectral range.

In accordance with an example arrangement, provided is a devicecomprising a nanowire having, a lower portion formed of a firstsemiconductor comprising at least a group III element and doped to ben-type, a central portion formed of the first semiconductor comprisingat least one quantum structure, and an upper portion of the firstsemiconductor comprising at least the group III element and doped to bep-type. The group III element may be gallium; and the firstsemiconductor material is gallium nitride. Further, the quantumstructure may be a quantum active layer structure formed of one or morequantum layers of Indium Gallium Nitride/Gallium Nitride (InGaN/GaN),such as one or more InGaN/GaN quantum dot layers.

In accordance with another example arrangement, provided is a devicecomprising a plurality of stand-alone nanowires collectively formed as acluster using a monolithically integrated fabrication technique, eachnanowire having a different diameter and each nanowire emitting aphotonic output at a different wavelength. An active layer in eachnanowire may have a different semiconductor composition from that ofeach other nanowire. The device may be a multi-colored solid-state lightsource. Each nanowire may comprise a quantum active layer structureformed of one or more quantum layers of Indium Gallium Nitride/GalliumNitride (InGaN/GaN), such as one or more InGaN/GaN quantum dot layers.Each nanowire may be a light emitting diode emitting at the differentwavelength.

In accordance with another example, provided is a method of fabricatinga semiconductor device, the method comprising: providing a substrate;depositing a metallic mask on the substrate, the metallic mask having apattern of spaced apart openings, each of a different size; growing aplurality of stand-alone nanowires through application of an epitaxialcrystal growth technique such that each nanowire has a differentdiameter determined by the size of one of the spaced apart openings; andforming each nanowire to have a quantum active layer structure, suchthat each nanowire is configured to emit a photonic output at adifferent wavelength corresponding to the diameter of the nanowire.

BRIEF DESCRIPTION OF THE DRAWINGS

This patent or application file contains at least one drawing executedin color. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the United States Patent andTrademark Office upon request and payment of the necessary fee.

FIG. 1A illustrates a monolithically integrated InGaN/GaN quantum layernanowire cluster, in accordance with an example.

FIG. 1B is an Scanning Electron Microscope (SEM) image of the InGaN/GaNquantum layer nanowire of FIG. 1A, showing with different nanowirediameters grown by selective area epitaxy, in accordance with anexample.

FIG. 2 illustrates a single InGaN/GaN nanowire grown on sapphiresubstrate, in accordance with an example.

FIG. 3 illustrates SEM images of InGaN/GaN nanowires of variousdiameters, in accordance with an example.

FIG. 4A is a plot of peak emission wavelength versus nanowire diameterfor a InGaN/GaN nanowire with a quantum dot active region grown at thethree different growth temperatures of 795° C. (Sample I), 810° C.(Sample II), and 825° C. (Sample III), in accordance with an example.

FIG. 4B is a plot of normalized Photoluminescence (PL) spectra versuswavelength for the InGaN/GaN quantum dot nanowires in Sample II, of FIG.4A, with different diameters measured at room temperature, showing ablue-shift in the emission peak with increasing nanowire diameter, inaccordance with an example.

FIG. 5A illustrates a process of atom adsorption to the surface of ananowire during a fabrication growth process. Note that the variation ofIndium (In) content is largely determined by diameters of each singlenanowire as shown schematically in FIG. 5B and FIG. 5C, in accordancewith an example.

FIG. 5B illustrates a process of increased Indium (In) adatomincorporation due to lateral diffusion for small diameter nanowires, inaccordance with an example.

FIG. 5C illustrates a process of reduced Indium (In) incorporation fromlateral diffusion for large diameter nanowires, in accordance with anexample.

FIG. 6A illustrates STEM-HAADF (Scanning Transmission ElectronMicroscope—High-Angle Annular Dark-Field) images for InGaN/GaN quantumdot nanowires with different diameters grown on a GaN template onsapphire substrate along the <1120> zone-axis.

FIG. 6B illustrates high-resolution STEM-EELS (Scanning TransmissionElectron Microscope—Electron Energy Loss Spectroscopy) maps of Indiumdistribution of active regions normalized to the sample thickness, inaccordance with an example.

FIG. 6C is a plot of relative In content versus InGaN layer locationderived from STEM-EELS (Scanning Transmission ElectronMicroscope—Electron Energy Loss Spectroscopy) analysis along differentlines in FIG. 6B, in accordance with an example.

FIG. 7A is a flow chart illustrating a monolithically integratedfabrication process, in accordance with an example.

FIG. 7B is another illustrated of a monolithically integrate fabricationprocess for the nanowire cluster of FIG. 1A. Note that the dimensionsand the various components are not drawn to scale here in FIG. 7B. Someof the dimensions have been exaggerated for the purpose of clarity andbetter demonstration of concepts. Only two nanowires are shown herebecause these are planar side views.

FIG. 8A is a schematic of a hole patterned titanium (Ti) mask showingthe presence of hexagonal openings with different sizes, in accordancewith an example.

FIG. 8B is a schematic showing selective area epitaxy of amonolithically integrated InGaN/GaN quantum layer nanowire cluster, inaccordance with an example.

FIGS. 9A-9E show the schematic illustrations showing the formation of asingle InGaN/GaN nanowire grown on GaN coated sapphire substrate. Theimages marked as FIGS. 9A to 9E illustrate an epitaxial crystal growthprocess corresponding to that of FIGS. 8A and 8B, in an example.

FIG. 10A illustrates four p-contact metallic electrodes attachedseparately and independently to four nanowires of an InGaN/GaN quantumlayer nanowire cluster, in accordance with an example.

FIG. 10B is a top-view of a single p-contact metallic electrode with therelative position of the Indium Tin Oxide (ITO) transparent electrode,in accordance with an example.

FIG. 11A is a plot of the current-voltage characteristics of InGaN/GaNquantum dot nanowires with different diameters, in accordance with anexample. The inset shows current density versus voltage in a semi-logplot, showing increasing leakage current for nanowire LEDs with largerdiameters.

FIG. 11B is a plot of the electroluminescence (EL) spectra versuswavelength for InGaN/GaN quantum dot nanowires with different diameters,in accordance with an example.

FIG. 11C is a plot of the light-current characteristics of InGaN/GaNquantum dot nanowires with different diameters, in accordance with anexample. The inset shows the EL spectra measured under differentinjection current densities (1.3-6.5 kA/cm2) for the green-emittingnanowires.

DETAILED DESCRIPTION

The present techniques include methods of fabricating solid-statesemiconductor nanowires and devices formed of the same. The nanowiresare novel InGaN/GaN quantum layer active region nanowires. Thesenanowires may be fabricated as photon emitting devices, such as lightemitting diodes (LEDs), or as photodetectors that absorb light atspecified wavelengths. Techniques herein further describe tuning thewavelength of the photon output of nanowires by fabricating them atdifferent diameter. Indeed, using a novel monolithically integratedfabrication process, a cluster of nanowires can be formed together intoa single, repeatable cluster of nanowires each emitting at different,tuned wavelength. The fabrication techniques are able to form theseclusters having nanowires of different diameters, each capable ofemitting at a different peak wavelength, such that the entire cluster isable to provide an output over a range of frequencies, such as over theentire visible spectrum. Each cluster may be formed as a single,repeatable chip like structure that may be fabricated in large scaleapplications, for example to form a digital display having a large arrayof these clusters.

The InGaN/GaN quantum nanowire devices may be formed of nanowirestructures of different diameters grown on the same substrate inone-step selective area epitaxy. Note that in this disclosure the terms“nanowire” and “nanowire structure” are used interchangeably.

Through detailed Scanning Transmission Electron Microscopy (STEM)studies, it is observed that the position, size, and composition ofInGaN quantum dots depend on the nanowire diameter. For small diameternanowires, quantum dots with high Indium (In) content are positioned atthe center of the nanowires and are vertically aligned along thevertical axis of the nanowire. With increasing nanowire diameter,however, the formation of quantum dots with reduced indium contentbecomes more dominant on the semi-polar planes.

By exploiting such unique diameter-dependent quantum dot formation, wehave shown that tunable emission across nearly the entire visiblespectral range can be realized from stand-alone InGaN/GaN quantum layeractive region nanowires, grown on the same substrate in a single epitaxystep. Those quantum layer active regions may be formed as quantumlayers. These quantum layers may be formed of quantum dots, quantumdiscs, quantum wells, quantum dots/disks/wells with the presence of ashell structure, or the combination of similar structures.

The techniques herein are able to form nanowires isolated from oneanother in that each nanowire in a cluster is spaced far enough apartfrom each other nanowire so that the formation of each nanowire isunaffected by the formation of the other nanowires. This spacingdistance allows the fabrication to grow each structure simultaneouslywith but independent of the growth of the other nanowires in thecluster. The result is that a single fabrication process forms nanowiresof different diameters, which has never been done before, as well asnanowires of different atom concentrations in the active region, whichtoo has never been done before.

A monolithically integrated cluster of stand-alone InGaN/GaN nanowiresof various sizes have been fabricated on the same substrate by thecrystal growth method of selective area epitaxy, also called selectivearea growth (SAG), using radio frequency Plasma-Assisted Molecular BeamEpitaxy (PA-MBE) technique which is one example of the epitaxytechniques that could be used to fabricate such structures. The epitaxyof the semiconductor layers takes place on an n-type GaN template onsapphire substrate with a thin (10 nm) titanium (Ti) layer beingemployed as the growth mask. Opening sizes in the range of 80 nm to 1.9μm (1900 nm) were created on the Ti mask by using electron-beam (e-beam)lithography and reactive ion etching techniques, which can lead to aprecise control of the diameters of the forming nanowires.

FIG. 1A shows the schematic illustration of a multi-color InGaN/GaNnanowire cluster 100A formed of stand-alone nanowires 121A, 121B, 121C,and 121D monolithically fabricated using a single-step selective areaepitaxy growth technique. FIG. 1A illustrates the cluster 100A aftercompletion of the fabrication process. In the illustrated example, thecluster 100A includes the four nanowires 121A, 121B, 121C, and 121D,although a cluster may be formed of any plurality of nanowire clusters(2, 3, 4, 5, 6, 7, 8, and so on).

Each nanowire 121A, 121B, 121C, and 121D has a different diameter fromeach other nanowire, thus establishing each nanowire as emitting at adifferent peak wavelength. However, in other examples, the cluster 100Amay be formed of at least two nanowires having the same diameter,depending on the application. For example, in some examples two or moreof the nanowires forming a cluster may have the same diameter and emitat the same wavelength. In some examples, multiple pairs of samediameter nanowires may be formed. Such clusters may be used, forexample, to further increase intensity ranges over certain wavelengthsemitted by a cluster.

In the cluster 100A, the nanowires 121A-121D are each hexagonalcross-section nanowires. With the present techniques, clusters may beformed of nanowires having cross sections of various shapes. Forexample, the cross sections could be cylindrical, hexagonal,rectangular, or triangular. The term diameter, as used herein, refer tothe effective diameter of the structure. For example, for a hexagonalcross section nanowire, like 121A-121D, the diameter is the average ofthe major and minor axis of the hexagonal cross section of the nanowire.

The nanowires 121A-121D are grown above a GaN coated sapphire substrate116. A GaN template layer 117 is deposited over the sapphire substrate116, and deposited on top of the GaN template layer 117 is a Titanium(Ti) patterned mask 114, also referred herein as Ti mask, which has aprecise set of openings on it. As discussed further below, the nanowire121A, 121B, 121C, and 121D are grown on the GaN template layer 117,extending only through the openings in Ti mask 114. The cluster 100A maybe packaged in a single chip device, for example, by applying apassivation layer 115 that fills the areas between the nanowire121A-121D and that also serves as a planarization surface.

Each nanowire 121A-121D is capped with dedicated p-contact metalliclaterally extending electrode 112A, 112B, 112C, and 112D, respectively,through which electric current is injected into each nanowire separatelyand independently. The electrodes 112A-112D provide independent controlof the injection current into each individual nanowire 121A-121D,thereby allowing independent current and voltage control of eachnanowire 121A-121D and independent control of parameters such asphotonic output intensity. Opposite the electrodes 112A-112D, ann-contact metallization electrode 113 is deposited on the GaN templatelayer 117. Once electric current is injected into the differentnanowires 121A-121D, each nanowire emits a photonic output 111A, 111B,111C, and 111D, respectively. In the illustrated examples, that photonicoutput is at different respective wavelengths, each over a differentregion of the visible spectrum. In an arrangement, the output 111A is at˜659 nm (red), the output 111B is at ˜625 nm (orange), the output 111Cis at ˜526 nm (green), and the output 111D is at ˜461 nm (blue). Theseoutput wavelengths are provided by way of example, and in particular, inan example implementation of the cluster as full color display pixel.With the present techniques quantum active layer nanowires may be formedto emit at wavelengths within a range of frequencies, such as overvisible wavelengths such as a violet spectral range of between at orabout 380 nm to at or about 450 nm, a blue spectral range from at orabout 450 nm to at or about 495 nm, a green spectral range from at orabout 495 nm to at or about 570 nm, a yellow spectral range from at orabout 570 nm to at or about 590 nm, an orange spectral range from at orabout 590 nm to at or about 620 nm, and a red spectral range from at orabout 620 nm to at or about 750 nm. These are provided by way ofexample, the output wavelengths may include wavelengths in the nearinfrared (˜700 nm to 2500 nm) and mid infrared, as well as in the nearultraviolet (˜380 nm to ˜450 nm), mid ultraviolet (˜280 nm to ˜380 nm),or deep ultraviolet (˜200 nm to ˜280 nm). When formed as aphotodetector, these wavelengths and spectral ranges would correspond toabsorption wavelengths.

FIG. 1B shows an Scanning Electron Microscope (SEM) image of fournanowires each with a different diameter, grown on a substrate using aTi mask as described further herein. The four nanowires 121A, 121B,121C, and 121D, have been grown on the GaN coated sapphire substrate 116using a crystal growth technique of selective area epitaxy, alsoreferred to herein as a selective area growth (SAG) technique. Thesubstrate does not necessarily need to be a sapphire substrate and couldbe other types of substrate such as Si, SiC, GaN, SiOx substrates andvarious metal substrates/templates. A more detailed discussion of anexample fabrication process is discussed in reference to FIGS. 7A-7Bbelow.

In FIG. 2 , a single nanowire 200, which may correspond to any of thenanowires 221A-221D, is shown. The nanowire is grown above a sapphiresubstrate 226 covered with a GaN template layer 227. In this illustratedexample, the nanowire 221 is formed of a n-type GaN layer 223, a stackof vertically aligned InGaN/GaN quantum layers 225, and a p-type GaNcapping layer 224. The quantum layers 225 may be formed of any number ofalternating InGaN and GaN layers, where the number of layers affects theoptical emission properties and where collectively these layers form anactive region of the nanowire 221. In some examples herein the InGaN/GaNquantum layers are formed of quantum dots, that is an InGaN dotstructure surrounded by a GaN or AIGaN filler. The layers 225 howevermay be formed of any number of quantum structures, including quantumdots, quantum discs, quantum wells, or similar structures. Furthermore,the nanowire 221 has a hexagonal (near perfect hexagonal) cross section.FIG. 3 , for example, illustrates SEM images of nanowires formed ofdifferent diameters each exhibiting near-perfect hexagonal morphologyand possessing Ga-polarity based on the terminating facets.

The nanowires in FIG. 3 were grown using a Veeco GENxplor MBE system,which is only one example of the type of system that can be used forthis purpose. The growth conditions, as an example, can include asubstrate temperature of 1030° C. and a gallium (Ga) beam equivalentpressure (BEP) of 3×10⁻⁷ Torr for the growth of the GaN segment of thenanowire. During process of growing the crystalline structure, thesubstrate temperature was reduced to 795° C., 810° C., and 825° C. forInGaN/GaN quantum dot active regions 25 in three different samples I,II, and III, respectively. In FIG. 4A the profiles 400A associated withthree samples I, II, and III are indicated by the numerical indicators442A, 442B, and 442C respectively. The growth temperature mentioned hererefers to the thermocouple reading near the backside (coated withmolybdenum and titanium) of the sapphire substrate. Indium and GalliumBEPs in the ranges of 1.2-1.5×10⁻⁷ Torr and 5-7×10⁻⁹ Torr were used forthe growth of the quantum dot active regions in accordance with theexample which is presented in this embodiment of the invention.

To confirm the output wavelengths emitted from these nanowires,photoluminescence (PL) emission was measured using a micro-PLmeasurement system at room-temperature with a 405 nm wavelength laser asthe excitation source. The plots 440A and 400B of these PL measurementresults are shown in FIGS. 4A and 4B

What is shown in FIG. 4A are the measured variations of the peakemission wavelengths versus nanowire diameters. Nanowires were grown onthe same substrate with identical epitaxy conditions, except that theirlateral sizes, i.e., diameters D, were varied in the range of ˜150 nm to˜2 μm (2000 nm) as shown in the SEM image of FIG. 3 . As shown in FIG.4A, the optical PL emission results show a consistent blue-shift withincreasing nanowire diameter under identical epitaxy conditions. Fornanowires in Sample II as an example, the emission wavelengths can becontinuously varied from a red region 443 of the spectrum at around thewavelength of ˜640 nm to a blue and violet region 441 of the spectrum ataround the wavelength of ˜465 nm by increasing the nanowire diametersfrom ˜150 nm to ˜2 μm (2000 nm) with otherwise identical epitaxyconditions. A similar trend is also observed for nanowires in Sample Iand Sample III, though the tuning range is less broad due to variationsin the growth conditions. The photoluminescence (PL) emission spectrafor nanowires with different diameters are further shown in FIG. 4B andit is evident that the wavelength range of emission covers the entirevisible range of the spectrum from violet all the way to red. Thesewavelength profiles have been shown in FIG. 4B by the numericalindicators 444A, 444B, 444C, 444D, 444E, 444F, and 444G.

The present techniques are able to advantageously control In, Ga, and Nconcentrations in the active regions of the nanowires and do so from asingle growth epitaxy process. FIGS. 5A-5C illustrate portions of thenanowire fabrication process that incorporate In and Ga atoms into agrown nanowire. FIG. 5A illustrates the process of atom adsorption,which is the adhesion of atoms, ions, or molecules from a gas to thesurface of a structure.

As shown, Indium (In) atoms 551 and Gallium (Ga) atoms 552 may beadhered to the nanowire 521A through an impingement upon an outersurface through the process of adatom (adsorbed atom) incorporation asshown in the illustrations of FIGS. 5A, 5B, and 5C. The In concentrationis largely determined by the diameter of the nanowire. FIG. 5B, forexample, shows a higher concentration of In in smaller diameternanowires, and FIG. 5C shows a lower concentration in larger diameternanowires. This process of adhesion of atoms to the surface structure ofthe nanowire formation is sometimes referred to as the “adatomincorporation process” with the word “adatom” meaning an atom that lieson the surface of a crystalline structure such as the nanowires beingdiscussed herein.

As illustrated in FIG. 5A, compared to conventional planar epitaxymethods in which only two dimensional planar layers of crystal areformed parallel to each other along the direction of the growth, theadatom incorporation process is different in crystalline structures thatinclude three dimensional formations such as the stand-alone nanowires.In the latter case, the crystal growth epitaxy process include atomsattaching themselves to the three dimensional structure through both aprocess of direct impingent atom adsorption from the top as well as theprocess of atoms migrating from the side and attaching themselves to theside walls of the nanowire from the lateral surfaces. In other wordsatoms get incorporated into the structure of the nanowire formation bothfrom the top surface of the nanowire as well as from the side surfacesof the nanowire.

For example, in reference to FIGS. 5A-5C, under relatively high growthtemperatures, Gallium (Ga) adatoms 552 (adsorbed atoms that attachthemselves to the surface of the crystalline nanowires 521A and 521D)have much larger diffusion lengths of around ˜1 μm (1000 nm) than Indium(In) adatoms 551 with diffusion lengths around ˜100 nm, with the latterlimited by thermal desorption. Since the Ga diffusion length iscomparable to, or larger than the nanowire diameters, it is expectedthat the Ga adatom 552 incorporation shows a small, or negligibledependence on nanowire size. However, significantly reduced Indium (In)incorporation is expected with increasing nanowire diameter, due to thereduced In adatom incorporation from lateral diffusion. In other wordsGallium (Ga) atoms penetrate the nanowire much more deeply than theIndium (In) atoms. The diffusion length, which refers to adatoms surfacemigration distance before they are either desorbed or incorporated inthe crystal, for In atoms is ˜100 nm while that of Ga atoms is ˜1000 nmwhich is about ten times more.

With increasing nanowire diameter, the reduced Indium adatomincorporation from lateral diffusion results in a reduced Indium contentin thicker nanowires, since the Indium beam equivalent pressure (BEP) isthe same across the entire wafer. As illustrated in FIGS. 5B and 5C,this means that nanowire that are relatively thicker will have an Indiumatom deficiency at their core as compared to nanowires which arerelatively thinner. For this reason, the composition of nanowires ofdifferent diameters will be different. As a result of different materialcomposition, thinner nanowires with a higher Indium content at theircore and will emit light of a shorter emission wavelength which will bemore towards the red side of the visible spectrum. Thicker nanowireswith a lower Indium content at their core will emit light of a longeremission wavelength which will be more towards the violet and blue sideof the visible spectrum. As shown in the particular example of FIG. 1A,the thicker nanowire 121D and 121C are able to emitting at bluewavelengths and green wavelengths, respectively, lights while thethinner nanowires 121B and 121A are able to emit at orange and red colorwavelengths.

In the case of the devices and the fabrication methods described herein,the variation of Indium content may be determined based on the diametersof the single stand-alone nanowires. This is because the nanowires arespaced relatively far from each other so that the formation of onenanowire during the crystalline growth process is not affected by itsneighboring nanowires.

These results presented herein in this disclosure are distinctlydifferent from those devices in which nanowires are formed in arrayswith high packing density and analogous to a forest with densely packedgrowth of trees spaced very near to each other. If the nanowires areformed in a densely packed array, as opposed to single stand-aloneensemble of nanowires as described here, then the growth and theformation of each nanowire is affected by what is commonly referred toas the “shadowing effect”. Under this alternate scenario theincorporation of both Indium and Gallium atoms into the structure of thenanowire is affected by the fact that the neighboring nanowire casts ashadow onto its neighboring nanowire and thus influences the mechanismthrough which Gallium and Indium atoms are diffused and incorporatedinto each nanowire. For this reason, in the formation of the devicesdiscussed in this disclosure, the nanowires are spaced relatively farfrom each other in a fashion that is shown for example in FIG. 1B.

To further elucidate the mechanism of wavelength tuning, investigated inthis work has been clusters of standalone InGaN/GaN nanowires withcontrollably varying spacing among the individual nanowires. Aconsistent red-shift with decreasing nanowire spacing is observed due tothe reduced Gallium incorporation into the structure of the nanowireformations. This is related to the beam shadowing effect. Such beamshadowing effect, however, is not present for single nanowires that arepart of a cluster of standalone nanowires. In other words, if thenanowires are spaced far enough from each other the undesirableshadowing effect and the resultant red-shift implication can be avoided.

A method of precisely controlling the spacing between the nanowires aswell as a method of precisely controlling the diameter of each nanowireis described in this disclosure. This control of the fabrication processis exerted through the use of a Titanium patterning mask and the methodof selective area epitaxy both of which will be described further onherein.

In order to identify the correlation between composition and structureof the InGaN/GaN quantum layer active region and the nanowire sizes,structural characterizations were performed using aberration-correctedScanning Transmission Electron Microscope (STEM) which can be done usingfor example an FEI Titan Cubed 80-300 STEM system that is operated at asetting of 200 kV. In doing so a cross-sectional sample of nanowires ofdifferent diameters was prepared by focused ion beam (FIB) technique ina single lift-out process, followed by a milling process done at asetting of 30 kV using for example a Zeiss NVision 40 dual-beam systemwith deposited Pt (platinum), C (carbon), and W (tungsten) films asprotection layers, and then perform a final polish of the sample at asetting of 5 kV. Having prepared the sample in this fashion it is thenpossible to produce STEM images of the nanowires and the core of thesenanowires. These images are shown in FIGS. 6A and 6B.

In FIG. 6A, the nanowires 621A-621D having active regions 625A-625D,respectively, each active region is a InGaN/GaN quantum layer activeregion and, more specifically, an active layer formed of stacks ofInGaN/GaN quantum dot layers. In the illustrated examples, the nanowires621A-621D have diameters of ˜320 nm, ˜420 nm, ˜500 nm, and ˜595 nm,respectively. As shown, the STEM structural analysis of FIG. 6A revealsthe nature and growth mechanism of InGaN/GaN quantum dot nanowires ofdifferent nanowire diameters, through the High-Angle Annular Dark-Field(HAADF) atomic-number contrast images.

FIG. 6B illustrates high-resolution elemental mapping of Indium usingSTEM Electron Energy-Loss Spectroscopy (EELS) of the active regions625A-625D corresponding to images 662A, 662B, 662C, and 662D. FIG. 6Billustrates Indium-distribution maps, identifying line profiles 1, 2, 3,and 4 (of active regions 625A-625D, respectively) which correspond tothe plot of In content versus InGaN layer location shown in FIG. 6C. Forthe Indium-distribution maps of FIG. 6C, the regions with brightintensity are rich in Indium content, while the regions with darkintensity are Indium-deficient or Indium-free. For example regions ofthe structure that are Indium free are the n-GaN region at the bottom ofthe nanowire and the p-GaN at the top of nanowire.

In FIG. 6B, in the TEM images 662A, 662B, 662C, and 662D the InGaNquantum dots are visible at the center of the nanowires. Shown in theimages 662A and 662B are the active regions 625A and 625B, which arevertically aligned along the vertical axis of the nanowire. Withincreasing nanowire diameter, instead of the formation of InGaN quantumdots at the center of the nanowires, InGaN accumulates at the semi-polarplanes of nanowires, as shown in active regions 625C and 625D of images662C and 662D, respectively. The larger the nanowire diameter, the moreInGaN is distributed at the semi-polar planes compared to the center ofnanowire which is the top of the n-GaN region. These observationsindicate that Indium incorporation at the nanowire top surface showsstrong dependence on nanowire diameter.

As shown, with increasing nanowire diameter the Indium content at thecenter of InGaN quantum dots decreases progressively. This result isconsistent with the blue-shift of the PL peak position with increasingdiameter of single nanowires, further supporting the growth mechanism ofsingle nanowires by using the selective area epitaxy.

FIGS. 7A and 7B illustrate a process 700A and 700B of fabricating anInGaN/GaN quantum layer active region nanowire device. This process isshown in the flow chart illustration of FIG. 7A and in the multipleillustrations of FIG. 7B. A GaN template layer 717 is formed on asubstrate 716 (701A and 701B), such as a sapphire substrate, for exampleusing epitaxial deposition. A Ti mask 714A is then deposited on the GaNtemplate layer (702A and 702B), using a metal deposition technique. At703A and 703B, the Ti mask is patterned to dedicated openingscorresponding to the location, shape, and cross-sectional dimensions ofthe various nanowires that will form a cluster. An electron beamlithography and reactive ion beam etching may be used to form the Timask pattern 714. Next, at 704A/704B, epitaxial deposition is performedto grow nanowires 712A and 712B, with active regions 725A and 725Brespectively, through the patterned Ti mask 714.

A polyimide passivation layer 715 is deposited at 705A/705B to fill theempty space between the nanowires. This process is followed by dryoxygen plasma etching of the top surface of the device to reveal the topsurface 715ES of the nanowires (706A and 706B). A metallization processforms a first/lower metallic layer 711A and 711B of p-contact electrodesand a subsequent annealing process is performed (707A and 707B). IndiumTin Oxide (ITO) transparent electrode 719A and 719B and an annealingprocess of electrodes is performed at 708A and 708B, followed by adeposition of metallic grids 721A and 721B onto the top surface of thetransparent ITO electrodes at 709A and 709B. A metallization processforming the n-contact electrode 713 is performed and an annealingprocess of the metallic contacts is performed (710A and 710B). Shown areoutput emissions 723A and 723B, each at a different wavelength.

As illustrated in FIGS. 9A-9E, once the patterned Ti mask layer 914,with precisely defined openings 914A, is formed on top of the GaN coatedsubstrate 926, the nanowires are grown in a step-by-step epitaxialcrystal growth process through which the various layers of the nanowireare grown one on top of the other. These various multiple layers of thenanowire are depicted in the illustration of FIG. 2 which depicts thenanowire at the conclusion of the epitaxial growth process. An exampleprocess of forming a nanowire is shown in FIGS. 9A-9E.

The process starts by first growing the n-type GaN crystal layer 923Aover the GaN coated substrate 926. The n-type GaN layer 923A is arelatively thick layer and in this example this n-type GaN layer is˜0.35 μm thick.

This relatively thick n-type GaN crystal layer 923A grows on top of theGaN coated substrate 926 only in the selected areas 914A that are notcovered (masked) by the Ti mask 914 and these selected areas are theopen holes 914A that have been created on the patterned Ti mask 914. Thecrystal layers of n-type GaN material 923A cannot take form over theareas of the substrate that are covered by the metallic Ti mask. Sincethe growth of the n-type GaN crystal takes place only over theseselected area openings the process is referred to selective area growth(SAG) epitaxial process. This concept is schematically illustrated inFIGS. 8A, 8B and FIGS. 9A-9E.

Continuing with the epitaxial crystal growth process, and following thegrowth of this relatively thick n-type GaN layer relatively, relativelythin layers of InGaN/GaN quantum layers 925 are grown one at a time overthe n-type GaN layer 923A. One thin layer of GaN is grown followed by athin layer of InGaN on top of it and then another thin layer of GaN ontop of this thin layer of InGaN. In this fashion, a multiple of InGaNand GaN layers 925 are grown one on top of the other just like thelayers of a multi-layered sandwich. For example 5 or 6 interchangingInGaN and GaN layers could be grown to form 5 or 6 layers of quantumstructures within which are embedded the quantum dot structures.

These multiple layers of InGaN/GaN comprise the active region 925 of thenanowire and they are neither n-type nor p-type, instead they are ofintrinsic (pure) semiconductor variety and these InGaN/GaN layers do notinclude any dopant within them.

Once the multiple InGaN/GaN quantum layers 925 have fully formed on topof the relatively thick n-type GaN layer 923, on top of these intrinsicInGaN/GaN quantum layers 925 is grown a relatively thick layer of p-typeGaN 924A, This thick p-type GaN layer 924A is sometimes referred to asthe capping layer because it is situated on the top of the stack oflayers. In this example this p-type GaN capping layer is ˜15 μm thick.As the entire nanowire grows vertically upward eventually at theconclusion of this crystal growth process the top of these nanowirestakes the form of a pyramid-shaped tips which are evident in the SEMimage of FIG. 1B (626A, 626B, 626C, and 626D) and the illustration ofFIG. 9E (924).

The thin multiple layers of intrinsic InGaN and GaN that form the activeregion of the nanowire are indicated in FIG. 2 and FIG. 9E collectivelywith the numerical indicator 925. Each of these thin InGaN/GaN layersare like thin disks that are stacked one on top of the other. However,moving from the center of the disk towards the outside perimeter of thedisk one sees that the composition of the material that comprises thisdisk changes. Moving from the center of the disk towards the outsideperimeter of the disk the concentration of Indium increases. In otherwords the areas of the disk that are closer to the perimeter of the diskare richer in the content of Indium and the areas closer to the centerof the disk are not as rich in terms of Indium concentration. Thisprocess is illustrated in the drawings of FIGS. 5A, 5B, and 5C.

The area near the center of each InGaN/GaN disk forms an Indiumdeficient core which resembles a disk-shaped elements embedded at thecore of the nanowire in the section that is comprised of InGaN/GaNlayers. These disk-shaped elements, which are embedded within the coreof the nanowire, form quantum dots. These quantum dots can takedifferent sizes and shapes, for example, disks, arch-shaped structures,semi-polar planes, wells, dots, dots within wells, dots with a shellaround them, spheres, or other similar forms and shapes or combinationthereof. The SEM images in FIG. 6A and FIG. 6B depict show examplequantum dot structures which in this example, are in the form of flatdisks, arches, and semi-polar planes (625A, 625B, 625C, and 625B in FIG.6A and FIG. 6B). That is, in the illustrated example, the nanowirehaving active region 625A is quantum dots that are flat-disk-shaped, asshown by 625A in both FIG. 6A and FIG. 6B. The slightly thickernanowires having active region 625B has quantum dots having an archshape. The yet thicker still nanowires having active regions 625C and625D have quantum dotes having a semi-polar shape.

The precise shape and composition of these InGaN/GaN quantum dots can bedesigned through the process of designing the diameter of the nanowire.How the choice of the diameter of the nanowire affects the formation ofthese quantum dots within the InGaN/GaN layers was described earlier andis schematically illustrated in the drawings of FIGS. 5A, 5B, and 5C.

The composition of the InGaN layers with various concentrations ofIndium is indicated by the term In_(x)G_(1-x)N. The proportion indicatorx changes within the InGaN/GaN quantum layers as one moves from thecenter of the nanowire towards the perimeter of the nanowire. Thischange of x indicates a variation in the spatial composition of thenanowire and is what results in the appearance of quantum dot shapedformations that are embedded at the core of the active region of eachnanowire.

A passivation layer of polyimide is applied (see, 705A and 705B in FIGS.7A and 7B, respectively). The passivation layer may be formed of anysuitable insulating polymer, other examples of which include SiOx, SiN,Al2O3, BN, etc. A dry oxygen plasma etching is performed (see, e.g.,706A and 706B) on the top surface of the passivation layer to reviewupper conduction surfaces of the nanowires. A metallization process isthen applied to the top of each nanowire in the cluster, for example, byapplying a p-contact conducting layer (or electrode), which may then beannealed (see, e.g., 707A and 707B). A Indium Tin Oxide (ITO) thin layermay then be deposited (see, e.g., 708A and 708B) to serve as atransparent electrode 719A or 791B and annealing process. Next ametallic grid layer is deposited on top of the ITO layer (see, e.g.,709A and 709B), and n-contact electrodes are formed through ametallization process (see, e.g., 710A and 710B), with annealing.

FIGS. 8A and 8B illustrate an example selective area epitaxy process forfabricating a full-color InGaN/GaN quantum nanowire cluster 800B, wheredifferent emission colors are achieved by varying the nanowire diametersin a single epitaxy step. In doing so, a patterned Titanium mask 814 hasbeen deposited on a Sapphire substrate 816 that is coated with a GaNtemplate layer 817. The openings 871 in the Titanium mask in thisparticular example are hexagonal shaped. Forming this patterned Titaniummask over Sapphire substrate structure 880A is the first part of theprocess of fabricating the nanowire structures.

In the illustrated example, the InGaN/GaN dot-in-nanowire LED structures821A-821D, each include a 0.44 μm thick layer of n-GaN 223, sixInGaN/GaN quantum layers each with quantum dot formations embeddedwithin them and collectively forming the active layer 225, and a 0.15 μmthick layer of p-GaN 224. The structures 821A-821D were grown in a VeecoGen II MBE crystal growth reactor system, which is one example of thetype of systems that can be used for the fabrication of the nanowires.During the crystal growth process substrate temperature was set to atemperature of 965° C. and the Ga beam equivalent pressure (BEP) was setto 3.1×10⁻⁷ Torr for Ge-doped GaN. The substrate temperature was reducedto 715° C. for the growth of the InGaN/GaN quantum dot active regions825. The temperature numbers mentioned here refer to the thermocouplereading. The Indium (In) and Gallium (Ga) BEPs used for the growth ofthe quantum dot active regions were 2.1×10⁻⁷ Torr and 3.2×10⁻⁹ Torr,respectively. The growth conditions for the Mg-doped GaN layer includeda Ga BEP of 3.1×10⁻⁷ Torr, Mg BEP of 1.86×10⁻⁹ Torr, and substratetemperature of 965° C., according with an example. Notice that thegrowth parameters are different from those used for single nanowirephotoluminescence (PL) studies that were described earlier, and this isdue to the use of a different MBE reactor system. Under these growthconditions in the Veeco Gen II MBE system, emission wavelengths acrossnearly the entire visible spectral range can be realized for nanowireswith diameters varying from ˜200 nm to ˜600 nm.

FIG. 8A shows the illustrations of a patterned Ti mask 814 with openingholes 871 with sizes of ˜150, ˜250, ˜350, and ˜550 nm is shown. Thelateral growth effect was also taken into account in the pattern design.As anticipated, due to this lateral growth effect, the diameter of thenanowires will be a bit larger than that the diameter of the holepatterns in the Ti mask. FIG. 8B shows the illustration of InGaN/GaNnanowires grown at four different diameters of ˜220, ˜320, ˜420, and˜630 nm (nanowires 821A, 821B, 821C, and 821D, respectively). Thenanowires have an average height of ˜650 nm, with near perfect hexagonalmorphology and smooth lateral surface, which can contribute to theenhanced light emission from the nanowire top surface. The nanowiresexhibit nearly maximum light extraction efficiency.

FIG. 10A illustrates a passivation and electrode formation that havebeen applied to the InGaN/GaN quantum nanowire cluster 1000A. Apolyimide passivation resist layer 1015 was spin-coated to fully coverthe nanowires 1021A-1021D, followed by oxygen plasma etching to revealthe top surface of nanowire. Nano-scale metal electrodes 1012A, 1012B,1012C, and 1012D, including Ni (7 nm)/Au (7 nm) metal layers 1011A-1011Dwere then deposited on the p-GaN top surface of individual nanowiresusing electron-beam (e-beam) lithography and metallization techniquesand then annealed at a temperature of ˜500° C. for 1 min in nitrogenambient. Subsequently, 100 nm thick Indium Tin Oxide (ITO) layers1019A-1019D were deposited to serve as a transparent electrode. Thecomplete devices with ITO contacts were annealed at a temperature of300° C. for 1 h in vacuum. The entire structure was formed on top of aSapphire substrate 1016 that is coated with a GaN template layer 1017 ontop of which is deposited the patterned Titanium mask 1014,

Note that the ITO layer is deposited on top of each nanowire by usinge-beam (or photo) lithography, such that p-contact electrodes toneighboring nanowires are not shorted. This way, each nanowire pixel canbe independently controlled.

Contact metal grids consisting of Ni (20 nm)/Au (100 nm) 1012A-1012Dwere then deposited on the ITO to facilitate electric current injectionand device testing. Subsequently, a Ti (20 nm)/Au (100 nm) n-metalcontact layer 1013 was deposited on the n-type GaN template 1017 andthen annealed at a temperature of ˜500° C. for 1 min in nitrogenambient.

FIGS. 10A and 10B illustrate four p-contact metallic electrodes(1012A-1012D which are collectively referred to as 1012) attachedseparately and independently to four nanowires (1021A-1021D which arecollectively referred to as 1021) of an InGaN/GaN quantum layer nanowirecluster, in accordance with an example. Each electrode has threeconductive layers. The first/lower layer is a relatively thin metalliclayer 1011A-1011D (collectively referred to as 1011), typicallycomprising of Ni (7 nm)/Au (7 nm) in accordance with an example. On topof this thin Ni/Au metallic layer is deposited an Indium Tin Oxide (ITO)transparent conductive layer (1019A-1019B which are collectivelyreferred to as 1019). On top of this ITO layer is deposited a metallicgrid (1012A-1012B which are collectively referred to as 1012), typicallycomprising of Ni (20 nm)/Au (100 nm) in accordance with an example. Thistypically thicker layer 1012 of Ni/Au forms the top layer of thep-contact metallic electrode.

These three conductive electrode layers (1011, 1019, and 1012), as theyare shown in FIGS. 10 and 10B, form the p-contact electrodes which aredeposited over the nanowire structures 1021A-1021B which in FIG. 10B arecollectively referred to as 1021. Also visible in this same drawing isthe Polyimide passivation layer 1015 which fills the empty spacesbetween the nanowire structures.

Experimentally obtained performance characteristics plots of singlestand-alone InGaN/GaN quantum dot nanowire LED devices were measuredunder continuous wave electrical biasing conditions at room-temperature.FIG. 11A shows representative current-voltage (I-V) curves of the blueemitting element (nanowire with diameter of D ˜630 nm), green emittingelement (nanowire with diameter of D ˜420 nm), orange emitting element(nanowire with diameter of D ˜320 nm), and red emitting element(nanowire with diameter D ˜220 nm), which exhibit excellentcurrent-voltage (I-V) characteristics. The nanowire LED devices haveturn-on voltages of ˜2 V, which is significantly better than previouslyreported ensemble nanowire LEDs and GaN-based planar devices.

FIG. 11A shows the current-voltage (I-V) characteristics curves (1191A,1191B, 191C, and 1191D) of single InGaN/GaN quantum dot nanowires withdifferent diameters. The inset of FIG. 11A shows current density versusvoltage curves (1192A, 1192B, 1192C, 1192D) in a semi-log plot, showingincreasing leakage current for nanowire LEDs with larger diameters.

FIG. 11B shows the electroluminescence (EL) spectra curves (1193A,1193B, 1193C, and 1193D) of single stand-alone nanowire LED elementseach with a different diameter. FIG. 110 shows the light-currentcharacteristics (L-I) curves (1194A, 1194B, 1194C, and 1194D) of singlestand-alone nanowire LED elements each with a different diameter. Theinset shows the EL spectra curves (1195A, 1195B, 1195C, and 1195D)measured under different injection current densities (1.3-6.5 kA/cm2)for the green-emitting single stand-alone nanowire LED element.

Current densities as high as 7 kA/cm² were measured at ˜3 V. It is alsonoticed that higher current densities can be achieved in nanowire LEDswith smaller diameters. This is largely due to the significantlyenhanced dopant incorporation in smaller diameter nanowires and theresulting efficient current conduction, as well as the more efficientheat dissipation. The capacity for sustaining higher current densitieswith decreasing device area has also been reported previously. Theseresults suggest that single nanowire optoelectronic and electronicdevices can handle unusually large current densities and can deliverextremely high power density compared to conventional planar devices. Asshown the leakage current under reverse bias is relatively small butincreases with increasing nanowire diameter, shown in the inset of FIG.11A as curves indicated by 112-A, 112-B, 112-C, and 112-D, which islikely due to the presence of defects in large diameter nanowires andthe resulting current leakage.

Single nanowire LEDs also exhibit excellent light emissioncharacteristics. The electroluminescence (EL) emission was collectedusing an optical fiber coupled to a high-resolution spectrometer anddetected by a charge coupled device (CCD). Shown in FIG. 11B are the ELemission spectra curves (1193A, 1193B, 1193C, and 1193D) of singlenanowire LED sub-pixels with diameters of ˜220 nm, ˜320 nm, ˜420 nm, and˜630 nm, which exhibit peak emission wavelengths of 659 nm, 625 nm, 526nm, and 461 nm, respectively. The spectra were taken at an injectioncurrent of approximately 4.5 μA. Light-current (L-I) characteristicscurves (1195A, 1195B, 1195C, and 1195D) of the red, orange, green, andblue single nanowire LED sub-pixels are shown in FIG. 11C. As shown thelight intensity increases near-linearly with injection current fordifferent nanowire LEDs. Stronger light intensity was measured fromnanowires with larger diameters under the same injection currentdensity, due to the larger active region area. On the other hand,nanowire LED sub-pixels with smaller diameters can handle higher currentdensity, due to the more efficient current conduction and heatdissipation.

Shown in the inset of FIG. 11C are the four EL spectra curves (1195A,1195B, 1195C, and 1195D) of the green-emitting nanowire LED pixel. Thereis no significant shift in the emission peak position with increasinginjection current, suggesting a small level of quantum-confinedStark-effect, due to the highly efficient strain relaxation ofnanowires. It is also worthwhile mentioning that, by engineering thenanowire diameter and height, single nanowire LEDs can offersignificantly higher light extraction efficiency and more controllableemission pattern, compared to conventional planar LEDs.

Moreover, it is further expected that, with the incorporation of p-AlGaNelectron blocking layer and core-shell schemes, the performance ofsingle InGaN/GaN nanowire LEDs can be dramatically improved by reducingnon-radiative surface recombination and carrier leakage and overflow.

In summary, the present techniques have demonstrated multicolor, singlestand-alone nanowire LED photonic devices on the same chip by using thespecial technique of single-step selective area epitaxy. Compared toconventional planar devices, such nanowire LED devices offer severaldistinct advantages, including significantly reduced dislocation densityand polarization fields, enhanced light extraction efficiency,controllable radiation pattern, tunable emission, and extremelyefficient current conduction and heat dissipation. Moreover, due to theextremely small size of these devices and reduced capacitance, suchnanowire devices also promise ultra-high speed frequency response. Themethods and devices demonstrated here provide a unique approach for therealization of tunable, full-color nano-scale optoelectronic devices fora broad range of applications, including ultra-fine imaging andprojection display, lighting, communication, sensing, and medicaldiagnostics on a single chip.

As used herein any reference to “one embodiment” or “an embodiment”means that a particular element, feature, structure, or characteristicdescribed in connection with the embodiment is included in at least oneembodiment. The appearances of the phrase “in one embodiment” in variousplaces in the specification are not necessarily all referring to thesame embodiment.

Some embodiments may be described using the expression “coupled” and“connected” along with their derivatives. For example, some embodimentsmay be described using the term “coupled” to indicate that two or moreelements are in direct physical or electrical contact. The term“coupled,” however, may also mean that two or more elements are not indirect contact with each other, but yet still co-operate or interactwith each other. The embodiments are not limited in this context.

As used herein, the terms “comprises,” “comprising,” “includes,”“including,” “has,” “having” or any other variation thereof, areintended to cover a non-exclusive inclusion. For example, a process,method, article, or apparatus that comprises a list of elements is notnecessarily limited to only those elements but may include otherelements not expressly listed or inherent to such process, method,article, or apparatus. Further, unless expressly stated to the contrary,“or” refers to an inclusive or and not to an exclusive or. For example,a condition A or B is satisfied by any one of the following: A is true(or present) and B is false (or not present), A is false (or notpresent) and B is true (or present), and both A and B are true (orpresent).

In addition, use of the “a” or “an” are employed to describe elementsand components of the embodiments herein. This is done merely forconvenience and to give a general sense of the description. Thisdescription, and the claims that follow, should be read to include oneor at least one and the singular also includes the plural unless it isobvious that it is meant otherwise.

This detailed description is to be construed as an example only and doesnot describe every possible embodiment, as describing every possibleembodiment would be impractical, if not impossible. One could implementnumerous alternate embodiments, using either current technology ortechnology developed after the filing date of this application.

What is claimed:
 1. A device, comprising: a substrate; a plurality ofnanowires coupled to and extending orthogonally from the substrate,wherein the nanowires are spaced distances apart from one another,wherein each nanowire of the plurality of nanowires comprises arespective active region and has a different respective effectivediameter, wherein each said respective active region comprises arespective quantum structure comprising a first group III element and asecond group III element, wherein the ratio of concentrations of thefirst and second group III elements in each said respective quantumstructure is dependent on the respective effective diameter of said eachnanowire, and wherein the plurality of nanowires comprises at least afirst nanowire that emits a first color of light and a second nanowirethat emits a second color of light that is different from the firstcolor; and a passivation layer that fills the distances between thenanowires.
 2. The device of claim 1, wherein the first nanowire and thesecond nanowire have different effective diameters.
 3. The device ofclaim 1, wherein atoms of the first group III element are disposedfurther into the nanowires than atoms of the second group III element,wherein the first nanowire has an effective diameter greater than aneffective diameter of the second nanowire, and wherein the firstnanowire has a concentration of the second group III element less than aconcentration of the second group III element in the second nanowire. 4.The device of claim 1, wherein the first group III element is gallium,and wherein the second group III element is indium.
 5. The device ofclaim 1, wherein each said respective quantum structure comprisesalternating layers of indium gallium nitride and gallium nitride.
 6. Thedevice of claim 1, wherein each said respective quantum structurecomprises layers of quantum dots, wherein the quantum dots in the secondnanowire are aligned along the longitudinal axis of the second nanowire,and wherein the quantum dots in the first nanowire are distributed in asemi-polar plane of the first nanowire.
 7. The device of claim 1,wherein each nanowire of the plurality of nanowires comprises an elementof a light emitting diode.
 8. The device of claim 1, wherein theplurality of nanowires comprises nanowires with a cross-section having ashape selected from the group consisting of: cylindrical, hexagonal,rectangular, and triangular.
 9. The device of claim 1, wherein the firstcolor and the second color are in respective spectral ranges selectedfrom the group consisting of: a blue spectral range; a red spectralrange; a green spectral range, and an orange spectral range.
 10. Thedevice of claim 1, further comprising: a plurality of electrodes,wherein each of the plurality of electrodes are coupled independently torespective ones of the plurality of nanowires opposite the substrate.11. A device, comprising: a substrate; and a plurality of nanowirescoupled to and extending from the substrate, wherein the plurality ofnanowires comprises a pair of adjacent nanowires formed in a same stepof a selective area epitaxial growth process, wherein the pair comprisesa first nanowire that emits a first color of light and a second nanowirethat emits a second color of light that is different from the firstcolor, wherein each nanowire of the plurality of nanowires comprises arespective active region and has a respective effective diameter,wherein each said respective active region comprises a respectivequantum structure comprising a first group III element and a secondgroup III element, and wherein the ratio of concentrations of the firstand second group III elements in each said respective quantum structureis dependent on the respective effective diameter of said each nanowire.12. The device of claim 11, wherein the first nanowire and the secondnanowire have different effective diameters.
 13. The device of claim 11,wherein atoms of the first group III element are disposed further intothe nanowires than atoms of the second group III element, wherein thefirst nanowire has an effective diameter greater than an effectivediameter of the second nanowire, and wherein the first nanowire has aconcentration of the second group III element less than a concentrationof the second group III element in the second nanowire.
 14. The deviceof claim 11, wherein the first group III element is gallium, and whereinthe second group III element is indium.
 15. The device of claim 11,wherein each said respective quantum structure comprises alternatinglayers of indium gallium nitride and gallium nitride.
 16. The device ofclaim 11, wherein each said respective quantum structure compriseslayers of quantum dots, wherein the quantum dots in the second nanowireare aligned along the longitudinal axis of the second nanowire, andwherein the quantum dots in the first nanowire are distributed in asemi-polar plane of the first nanowire.
 17. The device of claim 11,wherein each nanowire of the plurality of nanowires comprises an elementof a light emitting diode.
 18. The device of claim 11, wherein theplurality of nanowires comprises nanowires with a cross-section having ashape selected from the group consisting of: cylindrical, hexagonal,rectangular, and triangular.
 19. The device of claim 11, wherein thefirst color and the second color are in respective spectral rangesselected from the group consisting of: a blue spectral range; a redspectral range; a green spectral range, and an orange spectral range.20. The device of claim 11, further comprising: a plurality ofelectrodes, wherein each of the plurality of electrodes are coupledindependently to respective ones of the plurality of nanowires.